1. Field of the Invention
Aspects of the present invention relate to a liquid-gas separator for separating carbon dioxide from an unreacted liquid fuel discharged from an anode electrode of a direct liquid feed fuel cell.
2. Description of the Related Art
A direct liquid feed fuel cell is an apparatus that generates electricity through an electrochemical reaction between an organic fuel, such as methanol or ethanol, and an oxidant, i.e., oxygen. The electricity generated by a direct liquid feed fuel cell has a high energy density and a high current density. Also, since a liquid fuel, e.g., methanol, is fed directly to a direct liquid feed fuel cell, the direct liquid feed fuel cell does not require a peripheral device, such as a fuel reformer, and the liquid fuel is stored and supplied easily.
FIG. 1 is a cross-sectional view of a direct liquid feed fuel cell. Referring to FIG. 1, the direct feed fuel cell has a structure in which an electrolyte membrane 1 is interposed between an anode electrode 2 and a cathode electrode 3. The anode electrode 2 includes a diffusion layer 22 for supplying and diffusing a fuel, a catalyst layer 21 where an oxidation reaction of the fuel occurs, and an electrode supporting layer 23. The cathode electrode 3 also includes a diffusion layer 32 for supplying and diffusing the oxidant, a catalyst layer 31 where a reduction reaction occurs, and an electrode supporting layer 33. The catalyst layers 21 and 31 may each include a noble metal having superior catalytic characteristics at low temperatures, such as platinum. To avoid catalyst poisoning by CO, which is a by-product of the electrode reactions, a catalyst formed of an alloy of a transition metal, such as ruthenium, rhodium, osmium, or nickel, can be used. The electrode supporting layers 23 and 33 can be made of waterproof carbon paper or waterproof carbon fiber to easily supply fuel and discharge reaction products. The electrolyte membrane 1 is a hydrogen ion exchange membrane that has ion conductivity and can contain moisture. For example, the electrolyte membrane 1 may be a polymer membrane having a thickness of 50-200 μm.
An electrode reaction of a direct methanol fuel cell (DMFC), which is a type of direct liquid feed fuel cell, includes an anode reaction where fuel is oxidized and a cathode reaction where hydrogen and oxygen are reduced, as described below.
[Reaction 1]CH3OH+H2O→CO2+6H++6e−(Anode reaction)
[Reaction 2]3/2 O2+6H++6e−→3H2O (Cathode reaction)
[Reaction 3]CH3OH+3/2 O2→2H2O+CO2 (Overall reaction)
Carbon dioxide, hydrogen ions, and electrons are produced at the anode electrode 2 where the fuel is oxidized (reaction 1). The hydrogen ions migrate to the cathode electrode 3 through the electrolyte membrane 1. Water is produced by the reduction reaction between the hydrogen ions, electrons transferred from an external circuit, and oxygen at the cathode electrode 3 (reaction 2). Accordingly, water and carbon dioxide are produced as the result of the overall electrochemical reaction (reaction 3) between methanol and oxygen. Two moles of water are produced when one mole of methanol reacts with oxygen.
The liquid fuel used in the fuel cell may be a mixture of pure methanol and water produced in the system or already stored in the fuel cell. When a fuel of high concentration is used, the performance of the fuel cell is greatly reduced due to crossover of the fuel at the electrolyte membrane (the hydrogen ion exchange membrane). Therefore, methanol is typically diluted to a low concentration, such as 0.5 to 2 M (2 to 8 volume %).
FIGS. 2A and 2B are a cross-sectional schematic views of a conventional liquid-gas separator 10 used in fuel cells. When the liquid-gas separator 10 is at a normal position (refer to FIG. 2A), unreacted fuel and carbon dioxide enter the liquid-gas separator through an inlet 11. Carbon dioxide is exhausted from a hole 12 formed on a ceiling of the liquid-gas separator body, and the unreacted fuel is recovered to the fuel cell through an outlet 13 formed at a lower part of the liquid-gas separator body.
However, FIGS. 2A and 2B illustrate why a conventional liquid-gas separator 10, which may be suitable for a stationary fuel cell, is disadvantageous for a mobile fuel cell. Specifically, when used in a mobile fuel cell, the liquid-gas separator 10 is not fixed in one position and may become inverted. When the liquid-gas separator 10 is inverted (refer to FIG. 2B), carbon dioxide may enter the anode electrode through the outlet 13 and the unreacted fuel can be discharged to the outside through the hole 12.
Also, the liquid fuel that enters the inlet 11 of the liquid-gas separator 10 may include bits of catalyst, metal particles, and metal ions, such as, for example, Fe ions. The catalyst and metal particles can cause a malfunction of a pump (not shown) connected to the liquid gas separator 10, and the metal ions can contaminate the fuel stack.